The present invention relates to a photocurable adhesive composition, and in particular, a photocurable adhesive composition suitable for bonding of a liquid crystal panel.
A conventional cover panel for protecting a liquid crystal panel is slightly spaced from the liquid crystal panel. The cover panel and the liquid crystal panel are bonded by filling the space therebetween with an ultraviolet curable liquid resin, in order to make a product thinner and to enhance the brightness and the rigidity of the module. Examples of the ultraviolet curable resin include an active energy-ray-curable composition for optical materials containing a predefined amount of polyurethane (meth)acrylate that is produced by reacting a reaction product of isophoronediisocyanate and alkane diol at a predetermined ratio with (meth)acrylate having one hydroxyl group per molecule, and a predefined amount of a compound having a predetermined molecular weight and a (meth)acryloyl group (Japanese Patent Application Laid-Open 2007-23147). Unfortunately, the bonding process using such a liquid resin inevitably has a disadvantage in that air bubbles are trapped or the resin runs off from the edges when the liquid crystal panel is bonded with the cover panel, due to bumps on the cover panel formed by, for instance, printing.
There is also known the method of bonding a cover panel and a liquid crystal panel with a two-sided adhesive tape. However the two-sided adhesive tape, which has no flowability, cannot fill bumps on the cover panel formed by printing.
Alternatively, a method for bonding a cover panel and a liquid crystal panel using a photocurable adhesive sheet has been investigated. For instance, there is proposed a photocurable adhesive sheet being formed of a cationically photopolymerizable adhesive composition which comprises an acrylic polymer produced by copolymerizing an acrylic monomer with a radically polymerizable compound having a cationically polymerizable group, and a cationically photopolymerizable initiator (Japanese Patent Application Laid-Open HEI-11(1999)-116903).
However, the curing reaction by photo-cations has problems such as a low curing rate compared to photo-radical reaction and the inhibition of the curing by contained water. As a result, adhesion by a curable resin composition containing a photo-cationic curing system is not preferred for adhesion processes that require high production efficiency, such as bonding of a liquid crystal panel.
There is known an adhesive which employs a photo-radical reaction, for instance, an adhesive for electrically connecting and bonding two facing electrodes, which comprises a modified phenoxy resin having urethane bonds, urethane acrylate, and a radical reaction initiator (Japanese Patent Application Laid-Open 2007-9201). The adhesive is, however, not used for bonding a liquid crystal panel. As described in an example in Patent Literature 3, if the adhesive is applied to a liquid crystal panel as an adherend, it may severely damage the liquid crystal body due to its high curing temperature of 130° C. In addition, the urethane acrylate has a very small weight-average molecular weight of approximately 1,100.